1. The Field of the Invention
The invention relates generally to minimizing the effects of electromagnetic radiation in optical transceiver modules.
2. Background and Relevant Art
Today, communication systems using optical fiber as a means for transmission are widely employed for a variety of purposes, ranging from a basic transmission line in public communication channels to a short-distance network such as a LAN (local area network). Since most of the devices connected by these optical fibers are electronic devices rather than optical devices, optical transceivers are commonly placed at the interface between the optical fibers and the electronic devices.
An optical transceiver includes a transceiver substrate, such as a printed circuit board (PCB), that is connected to a transmitter unit and a receiver unit, and is enclosed in a module housing, or shell. The receiver unit, which usually includes a photodiode, receives optical signals and converts them into electrical signals for a host device. The transmitter unit, which includes a laser source such as a laser diode (or a light emitting diode), receives the electrical signals from the host device and converts them into optical signals. These converted signals can then be transmitted to yet another device. The receiver unit and the transmitter unit are often coupled to the transceiver substrate, and are generally referred to as a Receiver Optical Subassembly (ROSA) and a Transmitter Optical Subassembly (TOSA), respectively.
With reference to a TOSA, a component on the transceiver substrate, such as a laser driver component, sends an electronic signal through an alternating current (AC) “in” to the TOSA's laser source. The AC continues beyond the laser source and returns “out”, or back to the laser driver component, and then cycles back in again to the TOSA in a continuous cycle. As such, the AC loop can be thought of as having two AC circuit paths: an in path, and an out path. In conventional optical transceivers, particularly small form factor (e.g., SFF, SFP, and XFP) optical transceivers, these in and out paths are placed fairly close together.
At some point along the AC pathway, it is sometimes necessary to bias the AC by adding a direct current (DC, also referred to herein as “bias current”) to the AC on the way in to the TOSA. Unfortunately, it is also desirable to shield the laser driver, which drives the AC, from the DC bias levels. As such, DC is typically isolated from the AC by placing one or more capacitors along the AC in/out loop since the DC does not pass through a capacitor. Accordingly, the DC can be applied to the portion of the circuit connected to the TOSA, but not to the portion connected to the (laser) driver.
In addition, to a differential signal from the transceiver substrate to the TOSA, there is typically a certain amount of common mode signal along the same path. Simplistically, a common mode signal is a common signal appearing on two conductors that are intended to carry a differential signal. That is, if the voltage on path A increases when the voltage on path B decreases and vice versa, that is a differential signal, but if the signal on both A and B increases or decreases simultaneously, that is a common mode signal. In conventional transceivers, there is no circuitry set aside for providing a ground return for the common mode signal. In particular, the default return path for the common mode signal has been through the TOSA shell, and hence onto the transceiver chassis. This is because the TOSA shell is the closest (or only) return path in conventional transceivers, and also the path of lowest impedance.
Unfortunately, relatively high currents are now required to adequately drive the laser source component in the TOSA. Because such currents are switching at increasingly high frequencies, common mode electronic signals become increasingly problematic, particularly since the common mode signals simply propagate to the TOSA shell. In particular, common mode signals in high current systems can cause data errors, electromagnetic emissions above regulatory limits, and in some limited cases may result in damage to the TOSA and related optical transceiver components.
Electromagnetic radiation emitted by such signals, if not shielded properly, can result in cross-talk that interferes with operation of nearby optical and electronic devices. Such cross talk also becomes more problematic as optical transceivers shrink in size and the transceiver components are placed closer to each other. In particular, a ROSA is highly sensitive to any signal interference from cross talk since a signal received by a ROSA is highly amplified. One will appreciate therefore that high speed data transmissions present special challenges in an optical transceiver environment.
As such, an advantage in the art can be realized with systems that can appropriately nullify the negative effects of extraneous common-mode signals in high frequency data transmissions. Moreover, an advantage in the art can be realized with such systems that shield residual electromagnetic radiation between optical components, thereby improving data transmission and reception integrity.